Fareed,
As soon as you have taken our Final Exam and received your certificate for 35 Contact Hours, then you can go ahead and fill it in.
Why do you have to wait, even though you are already watching the PrepCast? This is a PMI requirement. PMI says in the PMP Handbook that the courses that you claim as your 35 Contact Hours must be completed, and your "proof" of completing the PrepCast is our certificate.
However, you purchased the PrepCast over 2 weeks ago, so if you feel ready, then you can take your final exam and get your certificate. Watch "E99.00 How to take your Final Exam" to see how it's done.